High bit rate digital optical transmission systems with standard glass fibers are predominantly used at present for long-range information transmission. In such cases linear and non-linear effects in the transmission fibers and system components limit the signal transmission range. The chromatic dispersion of the glass fibers in particular, also referred to as group velocity dispersion, abbreviated to GVD, results, as the data rate increases, in signal distortion and therefore shorter transmission ranges, if no measures are taken to compensate for dispersion. Non-linear effects, such as self-phase modulation, abbreviated to SPM, also cause signal distortion.
One option for compensating for such interference due to linear and non-linear effects consists of generating a predistorted signal on the transmit side of the transmission link, so that after propagation of the signal the interference on the link is eliminated on the receive side and the required signal form is present. This technique was first proposed by Koch et al. in the publication “Dispersion compensation by active predistorted signal synthesis”, Journal of Lightwave Technology, vol. LT-3, no. 4, August 1985. The patent document EP 0 654 193 dated 1993similarly refers to a pre-emphasis measure based on electrical pretreatment of the data signal, wherein both the optical amplitude and respectively output and the phase or frequency of the lightwave are modulated on the transmit side in such a manner that a practically undistorted data signal is present at the end of the dispersive transmission link.
In the patent publication WO2004/032385 A1 (hereafter referred to as document D1) and the US patent application with the publication number 2004/0067064 A1, arrangements are proposed for electrical precompensation for distortion caused by dispersion on an optical transmission link. The arrangements contain a processor component for precompensation 10, in which a digital filter 19, a digital/analog converter 22 and optionally also other filter arrangements are connected in series. In D1 the digital filter is realized from a serial to parallel converter (SPC) and a random access memory look-up table (RAM LUT). The digital filter is used to convert the uniquely distinguishable bit sequences of a data signal to a predistorted signal for compensation purposes according to a predetermined specification. This compensation signal is then supplied to a modulator. When the distortion of a transmission signal is caused by dispersion, each bit is influenced by its adjacent bits. The number of adjacent bits to be taken into account is less than 20 (or ±10) at 10 Gb/s and with standard single-mode fibers in the 1.5 μm wavelength range. The data signal is fed into an N-stage SPC, such as a shift register for example. The bit pattern, which is then present at the N pick-offs, is used as the address for the high-speed memory RAM-LUT of capacity 2N. Every address leads to a memory location, in which an associated value, calculated from the transmission function of the fiber section, is stored. The value associated with the respective address is then used in the (single or multiple) data clock pulse by way of a high-speed D/A converter and an amplifier to activate the I/O converter.
One disadvantage of this realization is that there are at present no RAMs, which can be read at the data rate of at least 10 Gbit/s and preferably higher than 20 to 40 Gbit/s. Also all possible data values that are a function of the bit pattern have to be stored, corresponding to 2N combinations.